1. Field of the Invention
The present invention relates to a lithium secondary battery having a pouch as the case thereof, and more particularly, to a secondary battery which has a pouch as a case thereof with a simplified safety device.
2. Description of the Related Art
In general, materials used for secondary batteries, such as positive and negative electrode active materials, a binder, an electrolyte, current collectors and the like, are electrochemically stable at the operating voltage of the batteries, ranging from 2.5 to 4.3 V. However, if the voltage of the battery is raised to over the operational voltage, the respective materials are decomposed at respective decomposition voltages producing gases. The produced gases increase the is internal pressure of the lithium secondary battery.
In the case of a lithium ion secondary battery using a metal can or a plastic as its case, a safety vent is provided for preventing ignition or explosion of the battery due to an increase in the internal pressure. In other words, in the case where gases are produced due to an increase in the internal pressure, that is, exceeding the operational voltage, or short circuiting, the safety vent is ruptured interrupting the flow of current.
Lithium batteries are generally known to be safer than lithium ion batteries. However, the lithium batteries are not still satisfactory in terms of safety. In other words, in the case where the internal pressure of a pouch increases due to gases produced by an increase in voltage exceeding the operational voltage, the pouch which is not so strong as metal, may swell due to an increase in the internal pressure and undergo a continuous increase in voltage. Then, lithium ions accumulate on a negative electrode side and precipitate lithium metal. Here, in the event that there is a minutely shorted portion or the lithium metal penetrates through a separator and comes into contact a positive electrode, a large amount of current flow and generates heat, thereby prompting generation of gases. Thus, a portion of the pouch finally leaks, causing ignition or explosion of the battery. Also, when the positive electrode and the negative electrode are externally shorted, rapid flow of current generates heat and produces gases. Likewise, ignition or explosion the battery occurs.
In order to avoid such adverse effects, in the prior art, over flow of current has been prevented by installing a protective circuit or a PTC (positive temperature coefficient) element, by which current flow is interrupted when the voltage reaches a constant level, during fabrication of unit cells. However, these elements are expensive, which causes an increase in the fabrication cost. Therefore, in a lithium secondary battery with a pouch, there has been increasing demands for low-cost safety devices for interrupting the flow of current when the battery operates abnormally.
To satisfy the above demands, it is an object of the present invention to provide a cheap and simplified safety device for a lithium secondary battery.
Accordingly, to achieve the above object, there is provided a lithium secondary battery encased in a pouch including a positive electrode terminal and a negative electrode terminal, both of electrodes extending upward with respect to the pouch, one of the electrode terminals having extension which wraps around the pouch, a lead end of the extension forming a new terminal wherein the extension includes a notch where the extension is narrower.
When a battery operates abnormally, that is, when a battery operates with a voltage higher than the operational voltage thereof or when a positive electrode and a negative electrode are shorted, gases are produced and the pouch swells accordingly. Then, the extended terminal which wraps around the pouch also swells. Due to swelling of the terminal, a stress is intensively applied to a notch of the terminal. As a result, excessive swelling of the pouch cuts the notch. Thus, the flow of current is interrupted to inhibit further reaction.